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Japan has scrambled fighter jets to see off what it believes were Chinese military planes 30 times in the last six months, according to officials.
This figure is more than twice the 13 times such an operation was carried out over the same period last year.
An official at Japan's Defence Agency (JDA) stressed that the offending planes had only approached Japanese airspace, not violated it.
But he said the Chinese were "expanding the scope of their operations at sea".
Relations between China and Japan have deteriorated over the last few months - damaged by rows over Japan's history textbooks, energy resources in the East China sea and the Japanese prime minister's repeated visits to a controversial war shrine.
The official told the BBC News website that the recent data, which tracked the number of times Japanese fighter jets were scrambled from April to September this year, included various assumptions, "because sometimes we could not identify [the foreign planes] but think they were Chinese".
Japanese airspace was never entered during this time, but the aircraft which were chased away were approaching Japanese territory, he said.
"We cannot make an analysis of what is behind this," he said, "but it is a fact that China is expanding their scope of operations at sea and the JDA will need to be attentive to the operations."
He said that the last time a foreign aircraft infiltrated Japanese military airspace was in 2001, and that it was a Russian plane. Russian aircraft accounted for about 90% of such incidents, he said.
Japan and Russia both claim islands off Japan's northern island of Hokkaido that were seized by Moscow near the end of World War II.
Japan has scrambled fighter jets to see off what it believes were Chinese military planes 30 times in the last six months, according to officials.
This figure is more than twice the 13 times such an operation was carried out over the same period last year.
An official at Japan's Defence Agency (JDA) stressed that the offending planes had only approached Japanese airspace, not violated it.
But he said the Chinese were "expanding the scope of their operations at sea".
Relations between China and Japan have deteriorated over the last few months - damaged by rows over Japan's history textbooks, energy resources in the East China sea and the Japanese prime minister's repeated visits to a controversial war shrine.
The official told the BBC News website that the recent data, which tracked the number of times Japanese fighter jets were scrambled from April to September this year, included various assumptions, "because sometimes we could not identify [the foreign planes] but think they were Chinese".
Japanese airspace was never entered during this time, but the aircraft which were chased away were approaching Japanese territory, he said.
"We cannot make an analysis of what is behind this," he said, "but it is a fact that China is expanding their scope of operations at sea and the JDA will need to be attentive to the operations."
He said that the last time a foreign aircraft infiltrated Japanese military airspace was in 2001, and that it was a Russian plane. Russian aircraft accounted for about 90% of such incidents, he said.
Japan and Russia both claim islands off Japan's northern island of Hokkaido that were seized by Moscow near the end of World War II.